1. Technical Field of the Invention
The present invention relates to a circuit for driving a display device, particularly to a display device for solving the image-retention phenomenon of a liquid-crystal display.
2. Description of the Related Art
As liquid-crystal displays (hereafter referred to as LCD) having larger sizes and higher definitions become available, their application is becoming common in displays for still images such as liquid-crystal displays used in computers and word processors as well as displays for moving images such as liquid-crystal displays used in TVs or the like. An LCD is slim compared to a TV having a CRT (Cathode Ray Tube) and it can be set without occupying a large space. Therefore, it is expected that more and more households will use LCDs. An LCD typically uses so-called AC driving to prevent liquid crystal deterioration, in which the LCD is controlled so that a DC-component voltage is not being applied to liquid crystal for a long period of time. To perform the AC driving, there is a method of alternately applying positive-polarity and negative-polarity signal voltages to a pixel electrode while keeping a voltage to be applied to a common electrode constant.
FIG. 1 is an illustration showing a configuration of an active matrix substrate of a conventional liquid-crystal panel. n (n is an integer) scanning lines 101 and m (m is an integer) signal lines 102 are arranged on the active matrix substrate and a TFT (Thin Film Transistor) 103 serving as a nonlinear device (switching device) is disposed near each of intersections of scanning lines 101 and signal lines 102.
The TFT 103 has a gate electrode connected to the scanning line 101, a source electrode connected to the signal line 102, and a drain electrode connected to a pixel electrode 104. The pixel electrode 104 constitutes a pixel capacitor 108 so as to interpose liquid crystal (not illustrated) between the pixel capacitor and a common electrode 105 disposed on an opposing substrate that faces the active matrix substrate.
The scanning lines 101 are connected to a scanning-line driving circuit 106 and the signal lines 102 are connected to a signal-line driving circuit 107. The scanning-line driving circuit 106 is operable to sequentially supply high potential to the n scanning lines 101 to turn on the TFTs connected to the scanning lines 101 as shown in FIG. 2. For a duration of scanning operation of the scanning-line driving circuit 106, the signal-line driving circuit 107 outputs a gray scale voltage VD corresponding to image data to any one of the m signal lines and thereby, supplying the gray scale voltage to the pixel electrode 104 through the turned-on TFT 103. The gray scale voltage serves to generate a potential difference between the common electrode 105 and the pixel electrode 104 to which a constant voltage is being applied and the potential difference generates an electric field so that the quantity of light passing through liquid crystal is controlled by an electric field, thereby resulting in display of image (Data denoted as <1> to <3> in FIG. 3 represents the pixel data in the first to third columns). Thus, the liquid-crystal panel is driven as shown in FIG. 4.
When displaying a moving image on the liquid-crystal display panel, currently, image-quality deterioration such as an image-retention phenomenon unfavorably occurs.
FIG. 5 shows how a speed at which liquid crystal responds to an image signal supplied thereto affects the brightness of the display panel. Because a speed at which a liquid-crystal material responds is low, when a gray scale voltage changes, liquid crystal cannot follow the change of gray scale voltage within one frame period and therefore, liquid crystal comes to response to the change over a several frame periods. This potentially causes the image-retention phenomenon. To solve the above problem, a variety of liquid crystal materials have been developed.
However, the report is conducted as follows by analyzing the aforementioned problem of image-retention phenomenon. That is, the study conducted by Japan Broadcasting Corporation Science and Technical Research Laboratory (for example, refer to the 1999 IEICE General Conference, SC-8-1, pp. 207–208) teaches that only the speed at which liquid crystal responds to an image signal is not responsible for occurrence of image-retention phenomenon, but the display scheme through which an LCD displays an image is also responsible for it. The problem found in the display scheme employed in an LCD will be described below by comparing the CRT driving method with the LCD driving method.
A liquid-crystal display is made to operate in accordance with the technique for sequentially driving lines in a direction from top to bottom lines as shown in FIGS. 2 and 3 and is a hold-type display device for holding a display image during one frame period. Because the liquid-crystal display device is operable to hold a display image during one frame period, a time difference occurs between a time interval during which an image is being displayed and a time interval during which a viewer moves its eyes on the image being displayed, causing an unclear image movement.
FIGS. 6(a) and 6(b) are presented to illustrate how a pixel of each of a CRT and an LCD emits light for image display in response to an image signal in the time domain.
As shown in FIG. 6(a), the CRT is the so-called impulse-type display device which emits light only for several milliseconds after an electron beam hits the fluorescent material on the surface of a tube. In contrast, the LCD shown in FIG. 6(b) is the so-called hold type display device for holding light for image display for one frame period ranging from the time when writing of data to pixels is completed to the time when the subsequent writing starts.
As shown in FIG. 6(a), when the CRT having the above characteristics and serving as an impulse-type display device displays a moving image, an object to be displayed is momentarily displayed at a position corresponding to the time at which the object is to be displayed. In contrast, when the LCD having the above characteristics and serving as a hold-type display device displays an image while keeping the image during one frame period, leaving the image until before beginning of writing of new data and causing an unclear image movement.
To prevent the unclear image movement, a liquid-crystal panel capable of quickly responding to an image signal has been developed and further, a driving method for displaying a moving image is disclosed in Japanese Patent No. 2000-122596 and the like. To prevent the unclear movement observed particularly in the hold-type display device, the driving method shown in FIGS. 7 and 8 is made available to the liquid-crystal active matrix substrate in FIG. 1.
The driving method shown in FIG. 7 or 8 is a method of resetting eyes and preventing an unclear image movement by inserting a black image during one frame period.
An image-retention phenomenon is avoided using the method in FIG. 7 or 8 comprising: writing image data to all the pixels of a certain pixel row as shown in FIG. 9; and at the same time, applying a black display voltage to all the pixels of another pixel row positioned apart a plurality of rows from the certain pixel row.
FIG. 10 shows an image displayed by driving liquid crystal using the method shown in FIGS. 2, 3 and FIG. 11 shows an image displayed by driving liquid crystal using the method shown in FIGS. 7, 8. As shown in FIG. 11, scanning a black display region is scanned over the screen eyes resets viewer's eyes and eliminates an unclear movement of moving image.
However, even if the an unclear movement of moving image is prevented by using the above signal-line driving method, the manufacture of a signal-line driving circuit still largely contributes to an increase in the cost of a liquid-crystal display even in a current situation in which there is strong requirement for cost reduction in the liquid-crystal display. Therefore, it is an important challenge to prevent an unclear movement of moving image and also reduce a signal-line driving circuit chip in size.
FIG. 12 shows the configuration of a conventional signal-line driving circuit. As shown in FIG. 12, the signal-line driving circuit is constituted by a shift register section 150, data register section 151, latch section 152, D/A converter section 153, and output buffer section 154. Image data is input through data buses (R0–R7, G0–G7, and B0–B7) and image data corresponding to the number of signal lines (image data corresponding to m pixels) are stored in the latch section 152. The stored image data corresponding to the signal lines are converted by the D/A CO converter section 153 into voltages adjusted to the transmittance performance of a liquid-crystal panel and output from the output buffer 154.
Symbol STH denotes a start pulse signal, HCK denotes a horizontal clock signal, STB denotes an output timing signal, POL denotes an output polarity inversion signal, and V0 to V9 each denote a reference gray scale voltage.
FIG. 13 shows the detailed output-section configuration of a signal-line driving circuit. Because positive-polarity signal voltage and negative-polarity signal voltage are alternately applied to a signal line, DAC+ for outputting a positive-polarity gray scale voltage indicative of image data and DAC− for outputting a negative-polarity voltage indicative of image data are arranged in the D/A converter section to realize AC driving by switching multiplexers 200 and 201 provided respectively in the latching section and output buffer section in response to a STB signal (or POL signal)
For example, the image data to be supplied to D1 is stored in the leftmost LAT in FIG. 13 and converted by the DAC+ or DAC−, which is determined by the multiplexer 200, and then, the image data is selected by the multiplexer 201 and output to D1 through an output amplifier 170. Note that the image data stored in the leftmost LAT never is output to D2.
Moreover, an output-section configuration of the conventional signal-line driving circuit may have the configuration shown in FIG. 14.
As described above, because the conventional signal-line driving circuit is constituted so as to hold the image data corresponding to signal lines (image data corresponding to m pixels) and then simultaneously output the image data to the signal lines, the number of outputs to signal lines substantially determines the size of signal-line driving circuit chip.
The techniques shown in FIGS. 7 to 9 still employ the method in which a signal-line driving circuit holds image data corresponding to signal lines and then outputs the data, thereby providing a configuration different from the scale-downed signal-line driving circuit configuration.